Secondary batteries in which charging and discharging are carried out through cations such as lithium ions passing between a positive electrode and a negative electrode are known. A typical example of such a battery is a lithium ion secondary battery. In such a secondary battery, an electrolyte in which a supporting electrolyte such as LiPF6 is dissolved in a nonaqueous solvent such as ethylene carbonate is often used.
On the other hand, molten salts that are able to be liquid in a temperature range in which a battery is used are known. Studies are being carried out into using an electrolyte having a composition containing such a molten salt as an electrolyte for a secondary battery as described above. However, with a battery in which such a molten salt-containing electrolyte is used, if the electric potential of the positive electrode becomes high, then the molten salt contained in the electrolyte becomes prone to undergoing oxidative decomposition. It has thus been difficult to raise the battery voltage of batteries containing such a molten salt-containing electrolyte.